1. Field of the Invention
The present invention relates to a printing plate precursor for laser engraving, a printing plate, and a method for producing a printing plate.
2. Description of the Related Art
As a method for producing a printing plate by forming concavities and convexities on a photosensitive resin layer laminated on the surface of a support, there is a well known method referred to as “analogue plate-making”, which comprises exposing a relief forming layer formed using a photosensitive composition to ultraviolet radiation through an original image film, selectively curing the image areas, and removing uncured portions using a developing solution.
The printing plate is an anastatic printing plate having a relief layer having concavities and convexities, and such a relief layer having concavities and convexities is obtained by patterning a relief forming layer that contains a photosensitive composition containing, as a main component, an elastomeric polymer such as a synthetic rubber, a resin such as a thermoplastic resin, or a mixture of a resin and a plasticizer, and forming concavities and convexities thereon. Among such printing plates, those having a flexible relief layer are referred to as flexo plates in some cases.
When the production of a printing plate is carried out by analogue plate-making, the production process generally requires an original image film utilizing a silver salt material, and thus, time and cost for the production of the original image film are required. Furthermore, since the development of the original image film requires chemical treatment and also requires developing waste disposal, simpler and easier plate production methods such as, for example, a method that does not use an original image film, a method that does not require a development process and the like, are under investigation.
In recent years, methods of carrying out plate-making of a relief forming layer by scanning exposure, without requiring the use of an original image film, have been investigated.
For a technique that does not require an original image film, a printing plate precursor having on a relief forming layer a laser-sensitive mask layer element that is capable of forming an image mask has been proposed. When a method for plate-making using such a precursor is applied, an image mask having a function similar to that of an original image film is formed from the mask layer element by laser irradiation based on image data, and therefore, such a method is referred to as a “mask CTP method”. This method does not require an original image film, but the subsequent plate-making process involves the steps of exposing the relief forming layer to ultraviolet radiation through the image mask, and developing and removing uncured portions. Thus, this method still has a room for improvement from the viewpoint that a development process is still required.
For a plate-making method that does not require a development process, a number of so-called “direct engraving CTP methods” have been proposed, in which plate-making is performed by directly engraving the relief forming layer with a laser. The direct engraving CTP method is literally a method of forming concavities and convexities which serve as a relief, by engraving with a laser. This method is advantageous in that, unlike a method of forming a relief using an original image film, the relief shape can be freely controlled. Therefore, in the case of forming an image such as an outline character, the image area can be engraved more deeply than other areas, while in the case of forming a fine halftone dot image, shouldered engraving can be adopted in consideration of the resistance to printing pressure. To date, a number of various plate materials have been proposed as the plate material to be used in the direct engraving CTP method (see, for example, Japanese National Phase Publication (JP-T) No. 10-512823, Japanese Patent Application Laid-Open (JP-A) Nos. 2001-328365 and 2002-3665, and Japanese Patent Nos. 3438404 and 2846955).
In the direct engraving CTP method, when a relief forming layer is directly subjected to plate-making with a laser, there is generated engraving waste that is formed of a low molecular weight polymerizable compound and the like. Since development waste remaining on the plate surface seriously affects the print quality, it is desirable to improve the removability of the generated engraving waste.
For the purpose of improving the removability of engraving waste, for example, International Publication (WO) No. 2004/00571 A1 pamphlet discloses a photosensitive resin composition for a laser-engravable printing plate precursor, which includes inorganic porous particles for adsorbing liquid waste.
Furthermore, JP-A No. 2002-244289 discloses an elastomer composition containing an elastomer, a monomer, a photoinitiator system in which the ultraviolet absorbance decreases as polymerization proceeds, and an additive that absorbs infrared radiation. JP-A No. 2002-244289 describes that an elastomer layer formed using an elastomer composition increases the engraving sensitivity and increases the speed of engraving, so that waste products (liquid waste) generated by engraving can be reduced.